A condenser microphone (sometimes referred to as capacitor microphone) consists of a transducer element that uses a very thin diaphragm spaced from an electrically charged backplate that together act like a variable capacitor to change acoustic energy into electrical energy. The acoustic energy vibrates the diaphragm, changing the spacing between the diaphragm and the backplate, changing the voltage maintained across them, which creates an AC electrical signal that corresponds to the acoustic energy vibrating the diaphragm. This signal is very small and weak and needs a circuit close by (usually housed inside the microphone along with the transducer), in order to amplify the signal to make it usable.
Small diaphragm pencil condenser microphones have long been a part of the recording industry, such as the Neumann KM84, AKG 451E, and many, many other microphones. Typically, what differentiates a pencil condenser from other recording studio condenser microphones is its small size and “front address” configuration. In a front address configuration, the microphone diaphragm is positioned to accept sound from the end of the microphone (such as a standard handheld microphone), whereas in a “side address” configuration, the diaphragm is positioned to accept sound from the side of the microphone (such as typical studio microphones, where the microphone is positioned vertically and the user speaks into the side).
Pencil condensers, due to their size, also use smaller capsule diameters—typically 15-22 mm in diameter, whereas typical studio microphones use capsules in the 30-34 mm range. The size of the capsule has several pros and cons when it comes to recording—transient response, frequency response, overall size of the microphone, sensitivity, and other factors—which make a range of diaphragm sizes useful in a high-fidelity recording environment.
The capsules on pencil condenser microphones are also sometimes interchangeable. The capsule is housed in a removable assembly on the front of the microphone, and the rest of the circuit—switches, electronic components, and connectors—are contained within the body of the microphone. A small diaphragm condenser is sometimes sold with several interchangeable capsules, each with a fixed polar pickup pattern. (The polar pattern affects the microphone's ability to accept or reject sound from different positions around the microphone.) An excellent example is the Avantone CK-1 microphone, which comes with three interchangeable fixed-pattern microphone capsules. Some front-address capsules can switch among patterns without being removed from the microphone, such as the Schoeps MK-5 capsule. To change the pattern in these microphones, components of the assembly are mechanically moved to change the way in which sound reaches the capsule. And, a very few pencil condenser microphones have interchangeable large capsule side-address assemblies, each with a fixed polar pattern, such as the Soyuz SU-0117 and the Shure Beta 181.